Trying to change modifier properties through script
You can get the list of existing modifiers from obj.modifiers.
You can access them by index or by name. For example, if the array modifier you want is the first in the stack and called "Array", you can get with either of these:


modifier = obj.modifiers[0]
modifier = obj.modifiers["Array"]


From there on you can set the properties as you see them in the Python tooltips or in the auto completion of the Python console (or the API reference).

One correct solution for the code of your example would be:

import bpy

obj =  bpy.context.object
modifier = obj.modifiers["Array"] # Get existing modifier with name "Array"
modifier.constant_offset_displace[0] = 1.6


Note that the user can change a modifier's name at any time, so you might want to use a more stable way of finding the modifier, but what's the best way to do that depends on your specific case.